Router Advertisement

On multicast-capable links and point-to-point links, each router periodically multicasts a router advertisement packet that announces its availability. A host receives router advertisements from all routers, building a list of default routers. Routers generate router advertisements frequently enough that hosts learn of their presence within a few minutes. However, routers do not advertise frequently enough to rely on an absence of advertisements to detect router failure. A separate detection algorithm that determines neighbor unreachability provides failure detection.

Router Advertisement Prefixes

Router advertisements contain a list of prefixes that is used for on-link determination. The list of prefixes is also used for autonomous address configuration. Flags that are associated with the prefixes specify the intended uses of a particular prefix. Hosts use the advertised on-link prefixes to build and maintain a list. The list is used to decide when a packet's destination is on-link or beyond a router. A destination can be on-link even though the destination is not covered by any advertised on-link prefix. In such instances, a router can send a redirect. The redirect informs the sender that the destination is a neighbor.

Router advertisements, and per-prefix flags, enable routers to inform hosts how to perform address autoconfiguration. For example, routers can specify whether hosts should use stateful, DHCPv6, or autonomous, stateless, address configuration.

Router Advertisement Messages

Router advertisement messages also contain Internet parameters, such as the hop limit that hosts should use in outgoing packets. Optionally, router advertisement messages also contain link parameters, such as the link MTU. This feature enables centralized administration of critical parameters. The parameters can be set on routers and automatically propagated to all hosts that are attached.

Nodes accomplish address resolution by multicasting a neighbor solicitation that asks the target node to return its link-layer address. Neighbor solicitation messages are multicast to the solicited-node multicast address of the target address. The target returns its link-layer address in a unicast neighbor advertisement message. A single request-response pair of packets is sufficient for both the initiator and the target to resolve each other's link-layer addresses. The initiator includes its link-layer address in the neighbor solicitation.

Neighbor Solicitation and Unreachability

Neighbor solicitation messages can also be used to determine if more than one node has been assigned the same unicast address.

Neighbor unreachability detection detects the failure of a neighbor or the failure of the forward path to the neighbor. This detection requires positive confirmation that packets that are sent to a neighbor are actually reaching that neighbor. And, that packets are being processed properly by its IP layer. Neighbor unreachability detection uses confirmation from two sources. When possible, upper-layer protocols provide a positive confirmation that a connection is making forward progress. Data that was sent previously is known to have been delivered correctly. For example, new TCP acknowledgments were received recently. When positive confirmation is not forthcoming through such hints, a node sends unicast neighbor solicitation messages. These messages solicit neighbor advertisements as reachability confirmation from the next hop. To reduce unnecessary network traffic, probe messages are sent only to neighbors to which the node is actively sending packets.

In addition to addressing the previous general problems, neighbor discovery also handles the following situations.

• Link-layer address change - A node that knows its link-layer address has been changed can multicast unsolicited, neighbor advertisement packets. The node can multicast to all nodes to update cached link-layer addresses that have become invalid. The sending of unsolicited advertisements is a performance enhancement only. The detection algorithm for neighbor unreachability ensures that all nodes reliably discover the new address, though the delay might be somewhat longer.

• Inbound load balancing - Nodes with replicated interfaces might want to load-balance the reception of incoming packets across multiple network interfaces on the same link. Such nodes have multiple link-layer addresses assigned to the same interface. For example, a single network driver can represent multiple network interface cards as a single logical interface that has multiple link-layer addresses.

  Load balancing is handled by allowing routers to omit the source link-layer address from router advertisement packets. Consequently, neighbors must use neighbor solicitation messages to learn link-layer addresses of routers. Returned neighbor advertisement messages can then contain link-layer addresses that differ, depending on who issued the solicitation.

• Anycast addresses - Anycast addresses identify one of a set of nodes that provide an equivalent service. Multiple nodes on the same link can be configured to recognize the same anycast address. Neighbor discovery handles anycasts by setting nodes to expect to receive multiple neighbor advertisements for the same target. All advertisements for anycast addresses are tagged as being non-override advertisements. Non-override advertisements invoke specific rules to determine which of potentially multiple advertisements should be used.

• Proxy advertisements - A router that accepts packets on behalf of a target address can issue non-override neighbor advertisements. The router can accept packets for a target address that is unable to respond to neighbor solicitations. Currently, the use of proxy is not specified. However, proxy advertising can potentially be used to handle cases like mobile nodes that have moved off-link. However, the use of proxy is not intended as a general mechanism to handle nodes that do not implement this protocol.

Comparison With IPv4

The neighbor discovery protocol of IPv6 corresponds to a combination of the IPv4 protocols Address Resolution Protocol (ARP), ICMP Router Discovery, and ICMP Redirect. IPv4 does not have a generally agreed on protocol or mechanism for neighbor unreachability detection. However, host requirements do specify some possible algorithms for dead gateway detection. Dead gateway detection is a subset of the problems that neighbor unreachability detection solves.

The neighbor discovery protocol provides a multitude of improvements over the IPv4 set of protocols.

• Router discovery is part of the base protocol set. Hosts do not need to snoop the routing protocols.

• Router advertisements carry link-layer addresses. No additional packet exchange is needed to resolve the router's link-layer address.

• Router advertisements carry prefixes for a link. A separate mechanism is not needed to configure the netmask.

• Router advertisements enable address autoconfiguration.

• Routers can advertise an MTU for hosts to use on the link. Consequently, all nodes use the same MTU value on links that lack a well-defined MTU.

• Address resolution multicasts are spread over 4 billion (2^32) multicast addresses, greatly reducing address-resolution-related interrupts on nodes other than the target. Moreover, non-IPv6 machines should not be interrupted at all.

• Redirects contain the link-layer address of the new first hop. Separate address resolution is not needed on receiving a redirect.

• Multiple prefixes can be associated with the same link. By default, hosts learn all on-link prefixes from router advertisements. However, routers can be configured to omit some or all prefixes from router advertisements. In such instances, hosts assume that destinations are off-link. Consequently, hosts send the traffic to routers. A router can then issue redirects as appropriate.

• Unlike IPv4, the recipient of an IPv6 redirect message assumes that the new next-hop is on-link. In IPv4, a host ignores redirect messages that specify a next-hop that is not on-link, according to the link's network mask. The IPv6 redirect mechanism is analogous to the XRedirect facility. The redirect mechanism is useful on non-broadcast and shared media links. On these links, nodes should not check for all prefixes for on-link destinations.

• Neighbor unreachability detection improves packet delivery in the presence of failing routers. This capability improves packet delivery over partially failing or partitioned links. This capability also improves packet delivery over nodes that change their link-layer addresses. For instance, mobile nodes can move off-link without losing any connectivity because of stale ARP caches.

• Unlike ARP, neighbor discovery detects half-link failures by using neighbor unreachability detection. Neighbor discovery avoids sending traffic to neighbors with which two-way connectivity is absent.

• Unlike in IPv4 router discovery, the router advertisement messages do not contain a preference field. The preference field is not needed to handle routers of different stability. The neighbor unreachability detection detects dead routers and dead switches to a working router.

• By using link-local addresses to uniquely identify routers, hosts can maintain the router associations. The ability to identify routers is required for router advertisements. The ability to identify routers is required for redirect messages. Hosts need to maintain router associations if the site uses new global prefixes.

• Because neighbor discovery messages have a hop limit of 255 upon receipt, the protocol is immune to spoofing attacks originating from off-link nodes. In contrast, IPv4 off-link nodes can send Internet Control Message Protocol (ICMP) redirect messages. IPv4 off-link nodes can also send router advertisement messages.

• By placing address resolution at the ICMP layer, the protocol becomes more media independent than ARP. Consequently, standard IP authentication and security mechanisms can be used.

IPv6 Stateless Address Autoconfiguration

A host performs several steps to autoconfigure its interfaces in IPv6. The autoconfiguration process creates a link-local address. The autoconfiguration process verifies its uniqueness on a link. The process also determines which information should be autoconfigured, addresses, other information, or both. The process determines if the addresses should be obtained through the stateless mechanism, the stateful mechanism, or both mechanisms. This section describes the process for generating a link-local address. This section also describes the process for generating site-local and global addresses by stateless address autoconfiguration. Finally, this section describes the procedure for duplicate address detection.

Stateless Autoconfiguration Requirements

IPv6 defines mechanisms for both stateful address and stateless address autoconfiguration. Stateless autoconfiguration requires no manual configuration of hosts, minimal (if any) configuration of routers, and no additional servers. The stateless mechanism enables a host to generate its own addresses. The stateless mechanism uses local information as well as non-local information that is advertised by routers to generate the addresses. Routers advertise prefixes that identify the subnet or subnets that are associated with a link. Hosts generate an interface identifier that uniquely identifies an interface on a subnet. An address is formed by combining the prefix and the interface identifier. In the absence of routers, a host can generate only link-local addresses. However, link-local addresses are only sufficient for allowing communication among nodes that are attached to the same link.

Stateful Autoconfiguration Model

In the stateful autoconfiguration model, hosts obtain interface addresses or configuration information and parameters from a server. Servers maintain a database that checks which addresses have been assigned to which hosts. The stateful autoconfiguration protocol allows hosts to obtain addresses and other configuration information from a server. Stateless and stateful autoconfiguration complement each other. For example, a host can use stateless autoconfiguration to configure its own addresses, but use stateful autoconfiguration to obtain other information.

When to Use Stateless and Stateful Approaches

The stateless approach is used when a site is not concerned with the exact addresses that hosts use. However, the addresses must be unique. The addresses must also be properly routable. The stateful approach is used when a site requires more precise control over exact address assignments. Stateful and stateless address autoconfiguration can be used simultaneously. The site administrator specifies which type of autoconfiguration to use through the setting of appropriate fields in router advertisement messages.

IPv6 addresses are leased to an interface for a fixed, possibly infinite, length of time. Each address has an associated lifetime that indicates how long the address is bound to an interface. When a lifetime expires, the binding, and address, become invalid and the address can be reassigned to another interface elsewhere. To handle the expiration of address bindings gracefully, an address experiences two distinct phases while the address is assigned to an interface. Initially, an address is preferred, meaning that its use in arbitrary communication is unrestricted. Later, an address becomes deprecated in anticipation that its current interface binding becomes invalid. When the address is in a deprecated state, the use of the address is discouraged, but not strictly forbidden. New communication, for example, the opening of a new TCP connection, should use a preferred address when possible. A deprecated address should be used only by applications that have been using the address. Applications that cannot switch to another address without a service disruption can use a deprecated address.

Duplicate Address Detection Algorithm

To ensure that all configured addresses are likely to be unique on a particular link, nodes run a duplicate address detection algorithm on addresses. The nodes must run the algorithm before assigning the addresses to an interface. The duplicate address detection algorithm is performed on all addresses.

The autoconfiguration process that is specified in this document applies only to hosts and not routers. Because host autoconfiguration uses information that is advertised by routers, routers need to be configured by some other means. However, routers probably generate link-local addresses by using the mechanism that is described in this document. In addition, routers are expected to pass successfully the duplicate address detection procedure on all addresses prior to assigning the address to an interface.